1. Field of the Invention
The invention deals with determining expected effects of transmission line systems.
2. Related Art
The speed in which digital systems can operate is often constrained by the transmission line effects of the printed circuit board (PCB) tracking. Since simulation techniques have not kept pace with advances in device switching speed, a designer may not be able to predict errors introduced by transmission line effects until a PCB has been prototyped. Furthermore, as time-to-market requirements are compressed, a designer is forced rely on simulations, as opposed to prototyping, to prove the functionality and robustness of a design. Therefore, in order to properly assess the impact of transmission line effects on digital system performance, simulation tools require an accurate method to incorporate such transmission line effects.
Previous techniques used in circuit simulators, for simulating transmission line effects for digital systems on PCBs, rely on non-physical approximations of the characteristics of either the digital devices or the transmission line network. For analog circuit simulators, such as SPICE, simulations can either be performed in the time domain or in the frequency domain. If the simulations are performed in the frequency domain, the transmission line system may be accurately described from measured Scattering (S)-parameters. However, linear approximations must be made for the termination characteristics of non-linear digital devices. If the simulations are performed in the time domain, the digital devices can be accurately described by transistor level models. However, the transmission line system must be approximated by either lumped elements or lossless delay line models.
Previous techniques used in circuit simulators for predicting the transmission line effects on PCBs with digital device termination have classically used one of the following methods. All of the these methods require non-physical approximations to the characteristics of either the transmission line system or the termination.
1) Modelling the transmission line effects of a PCB layout as a set of lumped element (L,R,C,G) subcircuits and simulating these circuits with nonlinear digital device terminations in the time domain.
2) Modelling the transmission line effects of a PCB layout as a set of lossless bi-directional delay lines and simulating with non-linear digital device terminations in the time domain.
3) Describing the transmission line effects of a PCB layout by the frequency domain S-parameters and simulating with linear approximations to digital device models in the frequency domain.
Lumped element models for transmission lines are only accurate for short electrical lengths at low frequencies. Multiple lumped element subcircuits are typically required in order to model delay. This technique tends to increase the both the cumulative errors and the simulation time of a circuit simulation due to the addition of multiple reactive elements.
Bi-directional delay line models for the transmission line effects of a PCB layout provide a significant improvement in accuracy at moderate frequencies over lumped parameter models. Unfortunately, a delay line cannot model discontinuities or any lossy or dispersive characteristics of a PCB. This is because the delay line models are based on infinitely long and straight transmission lines with no impedance discontinuities. In addition, depending on the implementation, there can be significant numerical convergence problems and accumulated numerical noise with this method.
Frequency domain solutions with an S-parameter description of the PCB layout provide very accurate solution of transmission line effects. However, linearization is a poor approximation for the termination characteristics of digital devices. Thus, frequency domain simulation does not return accurate results for digital systems.